In a cellular communication system each of the mobile stations communicate with typically a fixed base station. Communication from the mobile station to the base station is known as uplink and communication from the base station to the mobile station is known as downlink. The total coverage area of the system is divided into a number of separate cells each covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. As a mobile station moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the mobile station and the base station of the first cell to being between the mobile station and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a mobile station is routed through the fixed network to the destination specific for this call. If the call is between two mobile stations of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other mobile station currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a mobile station and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
Many different modulation methods are known for communication in cellular and other communication schemes. Some of these involve the transmission of data from a transmitter to a receiver over a plurality of distinct subchannels. Examples of this include Orthogonal Frequency Division Multiplex (OFDM) or multicode Code Division Multiple Access (CDMA) schemes. A common characteristic of these modulation schemes is that the transmitted power varies substantially dependent on the data in the subchannels resulting in a high peak to average ratio of the transmitted power.
A method of reducing this ratio is described in WO98/11698, which describes and OFDM system where ‘n’ bit data words are encoded as 2m symbol words, with the symbol words being generated to result in a desired low peak to average ratio. The document discloses introduction of error correction by increasing the size of the code word relative to the data word and transmitting the additional symbols over additional subchannels.
In order to limit degradation and spectral spreading of the transmission, a high peak to average ratio of the transmit power requires that the output power amplifier has to be linear over a wide dynamic range. This significantly impairs the cost and efficiency of the power amplifier. This is especially a problem in mobile communication systems where a low efficiency of the output amplifier significantly reduces the battery life of the mobile station.
Substantial benefits can thus be obtained by reducing the variations of the transmit power.